The present invention relates to an actuator and to a positioning device including an actuator.
In certain applications, it is necessary to accurately control the position of one part of a system relative to a second part, either to effect a relative displacement between the parts or to maintain a desired spatial relationship between the parts. For example, the cutting tip of a cutting tool is prone to wear, and requires periodic readjustment. Such readjustment can result in considerable downtime of the tool (in the order of 5%) with a consequent loss of production. Ceramics exhibiting the piezoelectric effect have been used as actuators for accurate positioning purposes, but the range of movement is small (about 5 xcexcm) and relatively large voltages are required.
An object formed from metal alloys known as shape memory effect (SME) alloys, can be programmed to memorise a particular shape (or shapes) by appropriate heat treatment. When the object (e.g. wire, plate or tube) is cooled below a transition temperature, it can be relatively easily deformed into any desired shape. When the object is heated above that transition temperature, the object forcefully returns to the programmed shape in the absence of an opposing force. This phenomenon arises from a change in state of the SME alloy. The temperature at which this occurs is dependent, inter alia, upon the composition of the alloy used. The force exerted by the object increases above the transition temperature to a maximum potential force at a given elevated temperature.
Such shape memory effect alloys have been used for many purposes, for example, as seals, circuit breakers, surgical stents and clamps. U.S. Pat. No. 4,899,543 discloses the use of a SME alloy-tube as an actuator for a clamp. The tube is located on a spindle between a retaining nut and a first clamping plate. The tube is bolted to the first clamping plate. When cooled, the tube lengthens and presses the first clamping plate against a second clamping plate. On heating, the tube shortens and moves the first clamping plate away from the second clamping plate.
It is an object of the present invention to provide an actuator assembly and positioning device which are capable of producing movement over a relatively large range and which are capable of low voltage operation.
According to a first aspect of the present invention, there is provided a positioning device, said device comprising
(i) an actuator constructed from a shape-memory effect (SME) alloy, said actuator having a pre-set memorised shape such that, from another shape, the actuator generates a first force when heated above a transition temperature;
(ii) heat supply means connected to said actuator;
(iii) urging means arranged to exert a second force acting in a direction opposite to the first force;
(iv) a positionable member moveable between first and second positions under the influence of said first and second forces;
(v) sensing means for detecting the position of the positionable member relative to a reference point; and
(vi) control means operably connected with said heat supply means and said sensing means for controlling the temperature of the actuator;
wherein, above an elevated temperature above the transition temperature, the positionable member is moved towards its first position against the action of the urging means, and wherein, below the elevated temperature, the positionable member is moved towards its second position by the urging means.
Preferably, the control means is arranged in use to cause the temperature of the actuator to be cycled above and below the elevated temperature. (Eg. cycled within a fraction of 1xc2x0 C. above and below the elevated temperature. It will be understood that this arrangement allows the actuator to be heated above and cooled below the elevated temperature relatively rapidly, thereby enabling a rapid response to whichever of the first and second forces is the greater.
Preferably the actuator is in the form of a plate. More preferably, the actuator has a pre-set memorised relatively planar shape. Most preferably, the actuator is arranged to move from the relatively planar shape to a relatively curved shape under the influence of the second force and from the relatively curved shape to the relatively planar shape under the influence of the first force.
The actuator may have a second pre-set memorised shape, such that, the actuator itself acts as the urging means when cooled below the transition temperature.
Preferably, the positioning device includes clamping means moveable between an open state and a closed state, said clamping means being arranged to clamp the positionable member at a desired position in its closed state, whilst allowing movement of the positionable member in its open state.
Preferably, the clamping means is resiliently biased into the closed state. More preferably, the clamping means includes an actuator constructed from an SME alloy which exerts a force sufficient to open the clamping means when said actuator is heated to an elevated temperature above its transition temperature.
Advantageously, the arrangement is such that the positioning device requires no power when the positionable member is clamped.
Preferably, the sensing means comprises a Hall sensor and a magnet. Conveniently, the magnet is arranged to move in correspondence with the positionable member, and the Hall sensor, located at the reference point, generates an output signal, the strength of which is dependent upon the distance of the magnet from the Hall sensor.
The urging means may conveniently comprise a spring (e.g. torsion, compression or diaphragm spring). A diaphragm spring is preferred.
According to a second aspect of the present invention, there is provided an actuator assembly comprising:
(i) first and second relatively moveable members;
(ii) an actuator plate constructed from a shape-memory effect (SME) alloy mounted between and in contact with said first and second members, said actuator plate having a pre-set memorised relatively planar shape, such that, from a relatively curved shape, the actuator plate generates a straightening force and moves towards its relatively planar shape when heated above a transition temperature; and,
(iii) heat supply means for adjusting the temperature of said actuator plate;
wherein, on raising the temperature of the actuator plate above the transition temperature in the relatively curved shape, the straightening force generated in the actuator plate acts on the first and second members to induce relative movement therebetween.
Preferably, the actuator plate has parallel opposite end surfaces which abut the first and second members respectively. More preferably, said end surfaces have an arcuately curved profile. Most preferably, the arcuately curved end surfaces are located in respective grooves of correspondingly arcuately curved profile provided in facing surfaces of the first and second members respectively. Such an arrangement permits smooth relative movement of the first and second members as the actuator plate moves between its relatively curved and relatively planar shapes.
Preferably, stop means are provided to prevent curvature of the actuator plate beyond a predetermined extent. More preferably no part of the actuator plate is subjected to a strain of more than 4% relative to the pre-set memorised shape.
Preferably, the heat supply means comprises an element (e.g. wire wrapped around the plate or heating element bonded to the surfaces of the plate) which is capable of distributing heat substantially evenly to the actuator plate.
The actuator plate can be constructed from nickel-titanium based alloy (xe2x80x9cNitinolxe2x80x9d) or copper-zinc-aluminium based alloy. Nickel-titanium alloy is preferred.
Preferably, the elevated temperature for the actuator plate is in the range 55 to 70xc2x0 C.
According to a third aspect of the present invention, there is provided a machine, a first part of which is to be positioned at a desired location relative to another part, said machine comprising a positioning device in accordance with the first aspect of the present invention and/or an actuator assembly in accordance with the second aspect of the present invention connected with said first part of the machine.
Said machine may be a machine tool.
According to a fourth aspect of the present invention, there is provided a method of positioning a positionable member, said method comprising the steps of:
(i) selecting a desired position for the positionable member between first and second positions;
(ii) causing a force to be applied to the positionable member when the positionable member is between the desired position and the second position as determined by step (iii), so as to urge the positionable member towards the first position;
(iii) sensing the position of the positionable member;
(iv) causing a force to be applied to the positionable member when the positionable member is between the first position and the desired position as determined by step (iii), so as to urge the positionable member towards the second position; and
(v) preventing movement of the positionable member when it has reached the desired position;
wherein, step (ii) is effected by heating an actuator constructed from a shape memory effect alloy above its transition temperature.
Step (v) may be effected by clamping the positionable member in the desired position.